Haptic actuators are devices that generate vibrations that can be felt by a person. Haptic actuators have become increasingly important in applications in handheld devices, such as cellphones and smartphones. Additional background information about haptic actuators is disclosed in U.S. Patent Application Publication No. 2011/0241851 to Henderson et al., which is herein incorporated by reference in its entirety.
However, there are some limitations to the maximum reaction force that prior art haptic actuators can produce in practical applications. In particular, the dynamic force (for a small size motor and/or moderate driven power) may not be sufficient to accelerate the entire mobile phone handset and create vibrations that are perceived by the user.
When a motor of small size (e.g., 6 mm in length and up to 2 mm in diameter) is subject to a force of about 20 grams of force (20 gf) at the node points (points on the motor that have the lowest vibration amplitude for a first bending mode vibration), the vibration amplitude of the motor begins to be dampened, and the maximum rotation speed of the shaft begins to decrease. Since the centripetal force is about 0.63 N (over 60 gf) for 200 Hz rotation of a typical rotating (Tungsten) mass of 0.4 grams offset about 1 mm from the centreline of the shaft rotation, and it acts upon antinode points of the motor (both ends or center which have the highest vibration amplitudes for a first bending mode vibration), this centripetal force will dampen the motor vibration even more than the 20 gf preload force at the node points of the motor. Thus, the maximum rotation speed of the shaft is limited (much below 200 Hz) and the resulting centripetal force is not sufficient for many applications.
Another potential limitation for the maximum reaction force is due to the way the motor is mounted. In the prior art, the motor is typically compliantly secured to a housing at the node points by an elastomer material, such as silicone. Unfortunately, the compliance of this mounting method will degrade the transmission of the centripetal force from the rotating unbalanced mass through the motor and then to the housing.